Two people have died and two more seriously hurt in a crash in the South Island near Dunedin, with three rescue helicopters flying in to treat the injured. 

A fifth person suffered moderate injuries. 

Emergency teams earlier rushed to the two-car crash on State Highway 1, just south of Waihola, after police were notified at about 9.15am - with a top cop citing the tragedy to remind Kiwis to stay safe on the roads this long weekend. 

St John said it sent three helicopters, three road ambulances and two manager vehicles. 

Three people were treated at the scene, it said. 

Two are now in a critical condition having been flown to hospital in separate helicopters, while a third person was taken in by road. 

SH1 is currently closed and diversions are in place, police said. 

"Northbound traffic is being diverted along Phosphate Road, and southbound traffic is being diverted through Taieri Mouth," police said. 

Police said the alternate routes that traffic is currently being diverted along is not suitable for trucks and heavy vehicles and these should expect lengthy delays. 

The Serious Crash Unit has also been advised. 

A Fire and Emergency New Zealand shift manager said four of its crews attended. 

Two others have already died in crashes during this long weekend period. 

A woman was killed when hit by a truck at Ōtāne in Central Hawke's Bay late yesterday afternoon, while a motorbike rider died in a crash with a car on State Highway 1 in Burnham, near Christchurch. 

Road policing director Steve Greally said each of these deaths is a tragic loss. 

"It's a person who should have had a future, they have family and friends, but unfortunately because of a mistake - whether it was them or somebody else - they're no longer with us, and the effects of that with their families, that's just huge and everlasting," he said. 

Greally urged drivers to follow basic road safety rules. 

This included making sure that drivers are not impaired by alcohol, fatigue or drugs, making sure that people aren't distracted by their cell phones, he said. 

"And of course making sure that people drive to the speed that's suitable - not only for the weather conditions, but also for the type of road, and no [faster] than the speed limit."